One way to increase the recording density of an optical disk is to employ a blue laser having a short wavelength (410 nm), as mentioned in Japanese Patent Laid-open No. 127174/1996. Another way to increase the recording density of an optical recording medium is to use an optical recording film of sulfide, oxide, or carbonyl of such metal as copper, platinum, gold, lead, chromium, molybdenum, and iron, which is capable of super-resolution, as mentioned in Japanese Patent Laid-open No. 267078/1994. The information recording layer of an optical disk may be formed from a material which changes from the crystalline phase into the amorphous phase, and vice versa, as mentioned in Japanese Patent Laid-open No. 198709/1997.
There is a continuing demand for an information recording medium capable of high density recording. A recording density as high as 1 Tbit/inch2 is required. One way to achieve this object is to employ a laser beam with a short wavelength which gives a spot having a small diameter. Increasing the recording density in this way is difficult because the reduction of laser wavelength is limited. A substantial increase in recording density is not expected from the recording system by super-resolution because it is only possible to reduce the laser beam diameter by half.
The present invention relates to an information recording medium and an information recording device. More particularly, the present invention relates to an information recording medium and an information recording device capable of high density recording.
It is another object of the present invention to provide an information recording medium and an information recording device capable of high density recording, accurate reproduction, and repeated recording and reproduction.
According to the present invention, the information recording medium has a recording film in which crystal grains are separated by an intergranular phase which is at the boundary between crystal grains. The advantage of this constitution is that the crystal grains undergo phase transformation in a range smaller than the beam diameter in the vicinity of the beam center and the intergranular phase prevents other crystal grains from being affected. The phase change in this manner accurately forms recording pits whose length is smaller than the laser beam diameter. Thus the information recording medium has a very high recording density.
According to the present invention, the crystal grains contain an oxide of Co, V, Mn, Fe, or Cu, and the intergranular phase contains an oxide of Si, Ti, Zn, Pb, or Bi, so that crystal grains undergo phase transformation upon irradiation with a laser beam and the intergranular phase prevents crystal grains from being affected. The crystal grains should preferably be those which undergo phase transformation between Co3O4 and CoO, Vo3O5 and V2O3, Mn2O3 and Mn3O4, Fe2O3 and Fe3O4, or CuO and Cu2O. The crystal grains should preferably be uniform in particle diameter. SiO2 existing in the grain boundary prevents the crystal grains from affecting other crystal grains, thereby contributing to high density recording.
The present invention is also directed to an information recording device which is made up of a recording film and an optical head, said recording film being composed of crystal grains and an intergranular phase existing at the grain boundary, said optical head being so designed as to supply the recording film with a near-field light whose diameter is made smaller than the wavelength of the laser beam. The fact that the optical head emits a laser beam of small diameter toward the recording film causes the phase transformation of crystal grains to take place in a small region, thereby permitting shorter recording pits to be made accurately. In addition, the fact that the intergranular phase prevents crystal grains from affecting other crystal grains permits the high density recording and accurate reproduction of information.